Computer systems perform a variety of functions including data acquisition, data processing and display of graphical images. The ability to integrate different external sources under control of a central processing unit generates a variety of applications for computer systems. For example, computers find applications in telecommunications where the display monitor provides a graphical display for input messages to an operator, and a user interface provides a means for the operator to generate output messages. In addition, computer systems are utilized for multi-media productions. In general, multi-media productions require integration of several audio and video production units to provide a single controllable unit. Multi-media projects cover many communication media types, including printed materials, audio programs, television shows, feature films and many others. Such computer system applications often require a single output monitor to display video data generated by an external source, and graphical data generated by the computer system.
As illustrated in the above examples, a computer system, configured to integrate computer graphics and external video, requires combining the video data and the graphics data on a single display monitor. In general, graphics adapters in computer systems that contain separate frame buffers for graphics data and video data may combine data in either the digital or analog domain. Although VESA connectors on graphics adapters permit merging graphics data streams with video data streams, VESA connectors exhibit physical limitations requiring 640.times.480.times.8 bpp output displays. In order to merge graphics data streams and video data streams for larger displays (e.g. 1024.times.768), operation in the analog domain is required.
Prior art methods for merging graphics data streams and video data streams in the analog domain require three 2:1 analog multiplexors, one for each primary color (e.g. red, green and blue). Typically, the three analog multiplexors are controlled by chroma keying logic that selects one of the two inputs based on detection of a color key. The color key is set to a black color defined by zero or a very low voltage on the analog red, green and blue signal lines. However, the use of a black key may cause undesirable side effects or artifacts when the computer system operates in a windows system environment. For example, if a drop down menu partially obscures or overlays a video window, any graphics pixels in the black chroma key color range are not displayed because the video window is selected in lieu of display of the black graphics. Consequently, the video shows through the black text in the menu. Typically, menus utilize black text, thereby rendering black a poor choice for the color key. Consequently, it is desirable to provide an analog chroma keying system that does not rely solely on black as the color key.